Suction pile anchors are commonly used in the offshore petroleum industry to moor offshore structures. A suction pile anchor can generally be described as a tubular element, typically but not exclusively circular in cross section, with a closed top and an open bottom. Current practice in the petroleum industry is to install suction pile anchors in a vertical orientation (i.e., the longitudinal axis of the tubular element is installed substantially perpendicularly to the sea floor) at a prescribed distance from the offshore structure it is anchoring. Methods for installing a suction pile anchor in a vertical orientation are known in the industry. In general, after being lowered to the sea floor, the suction pile anchor is allowed to penetrate the soil in a controlled descent, with the weight of the anchor being a primary driving force. Cables are used to help control the descent of the pile anchor, and pressure release mechanisms, such as two-way flow valves on the pile anchor, are opened to allow water to evacuate from inside of the pile anchor, thereby allowing penetration of the pile anchor into the soil. This process is referred to as self-weight penetration. Typically, pile anchors are installed into the seafloor by a combination of self-weight penetration, i.e. the weight of the anchor itself imbeds it into the seafloor, followed by the application of a force on the anchor to obtain the final desired depth of the anchor into the seafloor, i.e. its final penetration. Typically, this force is applied by way of suction penetration. In suction penetration, a water evacuation pump is attached to the suction pile anchor and water pumped out from the anchor's interior. The differential water pressure that is created results in a net downward force that is used to push the suction pile anchor to final penetration. A direct force can also be applied on the anchor, such as using a pile-driving hammer, to achieve final penetration. The direct force can be used either alone or in combination with suction penetration.
Typically, the pile anchor is connected to the offshore structure being moored by an anchor line. This connection is usually accomplished in either a catenary or taut-line application. In a catenary application, the anchor line is usually attached to the suction pile anchor near its top, with the anchor line resting on the sea floor except when tensioned by movement of the structure being moored. In a tautline application, the anchor line is always in tension and it is usually attached to the pile anchor at a location below the sea floor, a distance of some 60% to 70% of the suction pile anchor length, in order to obtain maximum anchor loading capacity. Taut-line suction pile anchor systems have been found to provide maximum resistance for typical deepwater applications where the anchor line assumes an angle with the horizontal equal to about 35 to 45 degrees.
In operation, failure of an anchoring device occurs when it no longer provides resistance against either vertical or horizontal loads, or a combination of both. Of particular concern for pile anchors is preventing vertical load failure. This concern is highlighted during the period after installation when the pile anchor is limited in its ability to resist vertical loads, and therefore the risk of vertical load failure is high. This behavior can be attributed mainly to installation induced disturbance (loss of soil shear strength) in a relatively thin zone of soil that surrounds the pile anchor. Analyses and experience show that the pile anchor can take several months after installation for the soil to regain essentially all of its strength. The lateral resistance is mostly unaffected by the thin zone of disturbed soil, because it mobilizes its resistance from the surrounding undisturbed soil. Accordingly, there is a need in the industry for an installation method that will reduce the risk of vertical load failure of a pile anchor.
When a pile anchor is installed in a traditional vertical orientation, the holding capacity of the anchor will typically increase with the size of the anchor. However, increasing the size of the anchor will cause a corresponding increase in material, fabrication and installation costs. Accordingly, there is a need in the industry for a method of installing a pile anchor that will allow the pile anchor to maintain the same holding capacity at a reduced anchor size. A pile anchor of reduced dimensions can also provide the added benefit of being installed to deeper penetrations, where the soils are typically stronger, and where even greater holding capacity can be achieved.
Correspondingly, there is a need in the industry to increase the holding capacity of a pile anchor of a given dimension in order to reduce the total number of pile anchors required to moor a floating structure. Decreasing the number of pile anchors reduces the installation time, which is a significant cost component of offshore construction.
Accordingly, there is a need for a method of installing a pile anchor that will reduce the risk of vertical failure and increase holding capacity of a pile anchor, and a corresponding need for an installation method that reduces the size and costs of the pile anchor. The present invention satisfies these needs.